The invention relates to a decoding circuit for digital samples defining N points of an initial signal transmitted by a transmitter by means of differential pulse code modulation DPCM at a rate determined by a clock H1 and each defining the absolute difference da encoded into p bits between two consecutive samples of the initial signal which digital samples are transcoded into r bits, where r is smaller than p.
This decoding circuit is used in receiver stages processing digital data stored in bulk memories or transmitted through radio channels.
A decoding circuit of this type is known from the book "Digital processing of speech signals" by L. R. Rabiner, R. W. Schafer, Prentice-Hall Inc Englewood Cliffs, New-Jersey, pages 208 etc. Differential pulse code modulation (DPCM) is a digital coding technique in which the difference of the values between two digitally coded adjacent samples are used for storage or for transmission. DPCM coding may thus be considered as a prediction coding in which the difference between a given value and a prediction is used. The variations between two successive digital data coded into q bits are determined and then represented by values transmitted in the form of digital samples which are transcoded in p bits where p is smaller than q in order to reduce the rate of digital information which is required for transmission.
The receiver circuit effects the inverse transformation, that is to say that it successively adds the value represented by the received digital samples da to the previously determined value. These samples da are encoded such that at the receiver stage the decoding circuit includes a transcoding member followed by an adder whose output is coupled to one of its inputs, the assembly operating at the same rate as that defined upon transmission.
The receiver must thus first reconstruct the signal after it has been filtered in order to render it usable. This involves a given complexity and necessitates a large number of logic functions particularly for filtering. The number of logic layers used at each operation leads to a given limitation of the working rate of the decoding circuit.
The initial signal which is defined by N points upon transmission is also obtained at N points upon reception after decoding. Generally this signal is subsequently smoothed by means of filtering for eliminating irregularities in restoring the signal. For treating digital signals it has been proposed to increase the restoring rate of output samples by generating supplementary samples. This is realized, for example in the document GB Pat. No. 2,078,406 but with different signals such as PCM signals. This document describes a linear interpolator based on a recursive filter structure and on a divide by N circuit in which the output of the interpolator operates at a rate which is N times higher than that of the input. But the circuit described is not used for DPCM signals at all.
The applications in which DPCM coding is used utilize the fact that this type of coding permits of considerably reducing the digital rate to be transmitted without degrading the restored signal to a too great extent, which is very important in the case of bulk memories or in radio channel transmission.